Debian Installation :: Remove All Sid Packages And Roll Back To Lenny?

I added Sid to my /etc/apt/sources.list, so that I could install KDE4 (as described on some random KDE4/Debian web page). However, I now think that was a mistake and it would have been better to attempt to backport KDE4 onto Lenny. Over time, more and more sid packages have crept into the system to resolve dependencies, and now my system seems a bit broken; 'top' won't run, complains about missing libtermcap which is there, my cron jobs seem to be ignored and so on.

So given a system that is a mix of Lenny and Sid, what is the best way to uninstall all the Sid packages and then get back to a clean Lenny install? Is there some way that I can list all Sid packages, maybe to a file, then use that to tell apt what Lenny packages to install after sources.list has been updated to remove Sid?

I kept a log as the system was built, of everything that had to be manually set up after the debian installer has completed when I moved from 32-bit to 64-bit, so I could just do a clean install of Lenny then reconstruct the system from scratch. However, would much prefer to use the package management tools to revert to a clean Lenny.